Physiological Basis for Inhibition of Morphine and Improgan Antinociception by Cc12, a P450 1 Epoxygenase Inhibitor 2

18 Many analgesic drugs, including mu-opioids, cannabinoids, and the novel non-opioid 19 analgesic improgan, produce antinociception by actions in the rostral ventromedial medulla 20 (RVM). There they activate pain-inhibiting neurons, termed " OFF-cells " , defined by a 21 nociceptive reflex-related pause in activity. Based on recent functional evidence that 22 neuronal P450 epoxygenases are important for the central antinociceptive actions of 23 morphine and improgan, we explored the convergence of opioid and non-opioid analgesic 24 drug actions in RVM by studying the effects of the P450 epoxygenase inhibitor CC12 on the 25 analgesic drug-induced activation of these OFF-cells and on behavioral antinociception. 26 In rats lightly anesthetized with isoflurane, we recorded the effects of intraventricular 27 morphine and improgan, with and without CC12 pretreatment, on tail flick latency and 28 activity of identified RVM neurons: OFF-cells, ON-cells (pro-nociceptive neurons), and 29 NEUTRAL cells (unresponsive to analgesic drugs). 30 CC12 pretreatment preserved reflex-related changes in OFF-cell firing and blocked the 31 analgesic actions of both drugs, without interfering with the increase in spontaneous firing 32 induced by improgan or morphine. CC12 blocked suppression of evoked ON-cell firing by 33 improgan, but not morphine. CC12 pretreatment had no effect by itself on RVM neurons or 34 behavior. 35 These data show that the epoxygenase inhibitor CC12 works downstream from receptors 36 for both μ-opioid and improgan, at the inhibitory input mediating the OFF-cell pause. This 37 circuit-level analysis thus provides a cellular basis for the convergence of opioid and non-38 opioid analgesic actions in the RVM. A pre-synaptic P450 epoxygenase may therefore be an 39 important target for development of clinically useful non-opioid analgesic drugs.